The present invention relates to semiconductors, and more specifically, to semiconductor structures having a gate stack.
Silicon dioxide has been used as a gate oxide material for decades. As transistors have decreased in size, the thickness of the silicon dioxide gate dielectric has steadily decreased to increase the gate capacitance and thereby drive current and device performance. As the thickness scales below 2 nm, leakage currents due to tunneling increase drastically, leading to unwieldy power consumption and reduced device reliability.
Replacing the silicon dioxide gate dielectric with a high-k material allows increased gate capacitance without the concomitant leakage effects. The term high-k dielectric refers to a material with a high dielectric constant (k) (as compared to silicon dioxide) used in semiconductor manufacturing processes which replaces the silicon dioxide gate dielectric. The implementation of high-k gate dielectrics is one of several strategies developed to allow further miniaturization of microelectronic components. Due to material compatibility issues, the use high-k gate dielectrics necessitates the use of metal gate electrodes rather than conventional polysilicon gate electrodes
One problem with utilizing high-k gate dielectrics with metal gate electrodes comes from increased parasitic capacitance (Cof) relative to conventional poly-silicon gate electrodes. In conventional poly-silicon electrodes, the poly-silicon depletion in the sidewall aids in lowering the COF. Therefore, migration to a metal gate incurs a Cof penalty.